Structure of switching device for liquid crystal display device and fabrication method thereof

ABSTRACT

A thin film transistor of an LCD device and a fabrication method thereof fabricates the thin film transistor of the LCD device including a source electrode unit of ‘U’ shape and a drain electrode of straight line shape formed as entering into a concave portion of the source electrode and a channel layer formed between the source electrode and a longer side of the drain electrode in fabricating the thin film transistor using a slit mask, and thereby, the LCD having even device property can be fabricated and inferiority such as short-circuit of the channel can be solved.

This application claims the benefit of Korean Patent Application No.2002-88472, filed on Dec. 31, 2002, which is hereby incorporated byreference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switching device of a liquid crystaldisplay device, and particularly, to a liquid crystal display deviceincluding a thin film transistor having a single-direction channel.

2. Discussion of the Related Art

Generally, a liquid crystal display (LCD) is an image display devicewhich is able to display desired information by supplying data signalsto pixels arranged in a matrix form according to information, andcontrols arrangement of the liquid crystal by an electric field.

Typically, the LCD includes: a liquid crystal panel on which unit pixelsare arranged in a matrix form and integrated circuits (IC) for driving aliquid crystal. The liquid crystal panel further includes a color filtersubstrate and a thin film transistor (TFT) array substrate, and theliquid crystal is filled in space between the color filter substrate andthe TFT array substrate.

In addition, on the TFT array substrate of the liquid crystal panel, aplurality of data lines for transmitting data signals supplied from datadriver ICs toward the unit pixels and a plurality of gate lines fortransmitting scan signals supplied from gate driver ICs to the unitpixels are crossed with each other in a right angle, and the unit pixelsare defined on the crossed portions of the data lines and the gatelines. The gate driver ICs supply the scan signals sequentially to theplurality of gate lines to activate the unit pixels one by onesequentially, and the data signals are supplied to the unit pixels of anactivated line from the data driver ICs.

On the other hand, a common electrode and a pixel electrode are formedon the color filter substrate and the TFT array substrate respectivelyfacing each other to apply an electric field to the liquid crystal. Thepixel electrode is formed on the TFT array substrate by unit pixels,while the common electrode is formed on the entire surface of the colorfilter substrate. Therefore, light transmittance of the liquid crystalcells can be controlled independently by controlling the voltage appliedto each of the pixel electrodes.

As described above, in order to control the voltage applied to each ofthe pixel electrodes, a TFT is formed on each of the liquid crystalcells as a switching device.

The components of the above LCD will be described in detail withreference to the accompanying figures.

FIG. 1 is a plane view showing a unit pixel of a general LCD. Referringto FIG. 1, gate lines 4 are arranged on a substrate in a row directionapart from each other, and data lines 2 are arranged in a columndirection apart from each other. A unit pixel is defined on everycrossed portions of the data line 2 and of the gate line 4, andcomprises a TFT and a pixel electrode 14. A gate electrode 10 of the TFTis formed as extended from a predetermined position of the gate line 4,and a source electrode 8, of which some part is overlapped with the gateelectrode 10, is extended from the data line 2. In addition, a drainelectrode 12 is formed on a position corresponding to the sourceelectrode 8, and the pixel electrode 14 is electrically contacted withthe drain electrode through a drain contact hole 16 formed on the drainelectrode 12. The thin film transistor (TFT) comprises a semiconductorlayer (not shown) for forming a conductive channel between the sourceelectrode 8 and the drain electrode 12 when the scan signals aresupplied to the gate electrode 10 through the gate line 4.

As described above, as the TFT forms the conductive channel between thesource electrode 8 and the drain electrode 12 in response to the scansignals supplied from the gate line 4, the data signals supplied to thesource electrode 8 through the data line 2 are transmitted to the drainelectrode 12.

The pixel electrode 14 connected to the drain electrode 12 through thedrain contact hole 16 is made of a transparent material, such as IndiumTin Oxide (ITO). At that time, the pixel electrode 14 generates anelectric field on the liquid crystal layer in association with a commontransparent electrode (not shown) formed on the color filter substratein accordance with the data signals supplied from the drain electrode12.

When the electric field is applied to the liquid crystal layer asdescribed above, the liquid crystal molecules rotate due to dielectricanisotropy to transmit the light emitted from a backlight toward thecolor filter substrate through the pixel electrode 14, and the amount ofthe transmitted light is controlled by the amount of the voltage of thedata signals.

In addition, a storage electrode 20 connected to the pixel electrode 14through a storage contact hole 22 is deposited on the gate line 4 toform a storage capacitor 18, and a gate insulating layer (not shown)which is deposited during the forming process of the TFT is formedbetween the storage electrode 20 and the gate line 4. The storagecapacitor 18 is to maintain operation of liquid crystal by chargingvoltage during the turned-on period of the TFT when the scan signal isapplied to the gate line 4 and by supplying the charged voltage to thepixel electrode 14 during the turned-off period of the TFT.

FIG. 2 is a cross sectional view of a unit pixel taken along a line I–I′in FIG. 1, and includes a color filter substrate 60 facing the TFT arraysubstrate 50 and attached with it; a spacer 70 separating the TFT arraysubstrate 50 from the color filter substrate 60 with a predetermineddistance therebetween; and a liquid crystal layer 80 which is the liquidcrystal filled in the separated space between the TFT array substrate 50and the color filter substrate 60.

The TFT (T), the switching device, and the storage capacitor (C) areformed on the TFT array substrate. The TFT is an essential device fordriving the liquid crystal and is fabricated in a process using fivemasks presently.

The process using five masks will be described with reference to FIG. 3as follows. FIGS. 3A˜3E are views illustrating the processes forfabricating the TFT using the 5 masks.

First, as shown in FIG. 3A, an electrode material 302 for forming a gateline is formed on a glass substrate 301. The electrode material forforming the gate line is generally a metal layer, and also functions aslines on a storing area for maintaining a voltage for a predeterminedtime and as a gate pad unit.

After forming the gate metal layer, a photoresist (not shown) isdeposited on the metal layer, and a photolithography process isperformed using a first mask (not shown) to form selectively a channelarea, a storing area and a gate pad unit pattern 302 on the glasssubstrate 301.

Next, as shown in FIG. 3B, a gate insulating layer 303 of SiNx, anactive layer 304 and a conductive layer 305 are formed sequentially onthe substrate. Then, a photolithography process is performed using asecond mask (not shown) to etch the active layer 304 and the conductivelayer 305 selectively so that the active layer 304 is defined as thechannel area. At that time, the active layer 304 is formed by depositingamorphous silicon (a-Si) and a high doped n-type layer. Plasma enhancedchemical vapor deposition (PECVD) method is generally used to depositthe insulating layer (SiNx) and the active layer.

Next, as shown in FIG. 3C, a photoresist (not shown) is coated on theconductive layer 305 and then, a photolithography process is performedusing a third mask (not shown). As a result of the above process, a partof the conductive layer 305 on an active area on which a channel of theTFT is formed is removed and the source/drain electrodes 306 and 307 aredefined. Also, a portion of the conductive layer 305 is selectivelyetched so as to form a data pad unit through which the data signal isapplied to the TFT.

Next, as shown in FIG. 3D, a passivation layer 308 is formed on theresultant surface, and then, a photolithography process is performedusing a fourth mask (not shown) to selectively etch the passivationlayer 308 so as to expose a part of the drain electrode 307 to form acontact hole.

Next, as shown in FIG. 3E, an electrode material is formed on theresultant surface, and a photolithography process is performed using afifth mask (not shown) to form a pixel electrode 309 connected to thedrain electrode 307.

The number of masks is an important factor for fabricating the LCD, andaffects directly to the cost or productivity. Therefore, a fabricationmethod of the LCD using 4 masks has been suggested. In the process offabricating the TFT using the four masks, it is important to form a halftone photoresist having different thickness by exposing differentlydepending on the positions of the TFT using a slit mask.

The related art slit mask structure will be described with reference toFIG. 4A. The slit mask comprises a source pattern unit 401 formed as ‘U’shape to increase a channel length shown in FIG. 4A, a drain patternunit 402 entered into a concave portion of the source pattern unit andseparated by a predetermined distance from three sides of the concaveportion, and a slit pattern unit 403. The slit pattern unit 403 furtherincludes a horizontal unit 403 a formed between the source pattern unitand the drain pattern unit and horizontal with a long side of the drainpattern, a vertical unit 403 b which is vertical to the long side of thedrain pattern, and a slant unit 403 c where the horizontal unit and thevertical unit meet.

In the four-mask TFT process using the slit mask, the interval of slitand the pattern shape are important factors for determining a shape ofthe channel and accuracy. In the process of fabricating the pattern ofthe slit mask, the slit pattern must be fabricated in a precise way witha tolerance less than 2 μm. However, as shown in FIG. 4A, since therelated art mask is formed as ‘U’ shape with the slant unit where theslit pattern is bent, the pattern interval becomes uneven especiallyaround the slant unit. Even if the intervals between slits in the maskare the same, variation between the horizontal unit and the slant unitis generated due to technical limits of the mask during fabrication.

Generally, the difference between the width of a channel formed by thehorizontal unit of the slit pattern and the width of a channel formed bythe vertical unit of the channel is within 500 Å, while the differencebetween the width of the channel formed by the horizontal or verticalunits of the slit pattern and the width of the channel formed by theslant unit of the slit pattern is about 2000 Å. FIG. 4B is a scanningelectron microscope (SEM) photo showing a shape of an inferior channeldue to the accuracy inferiority of the slit mask.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a switching device ofa liquid crystal display device that substantially obviates one or moreof the problems due to limitations and disadvantages of the related art.

An advantage of the present invention is to provide a thin filmtransistor having an active layer of a single direction by applying aslit mask on which slits are arranged toward the single direction inorder to fabricate a thin film transistor of a liquid crystal displayhaving even channel widths.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, a thin filmtransistor of a liquid crystal display (LCD) may, for example, include asource electrode of ‘U’ shape, a drain electrode formed on a concaveportion of the source electrode and a channel layer formed between thesource electrode and a long direction of the drain electrode.

In another aspect of the present invention, a method of fabricating aswitching device in an LCD may, for example, include forming a gateelectrode on a substrate, forming a gate insulating layer, an activelayer and a conductive layer sequentially on the substrate, forming asource electrode of ‘U’ shape and a drain electrode formed on a concaveportion of the source electrode, forming a channel layer between a longdirection of the drain electrode and the source electrode, and forming apassivation layer and a pixel electrode.

In another aspect of the present invention, a method of fabricating athin film transistor for a liquid crystal display may, for example,include forming a gate electrode on a substrate; forming a gateinsulating layer, an amorphous silicon layer, a high doped n type layerand a conductive layer sequentially on the substrate; formingsource/drain electrodes and a channel area using a slit mask; andforming a pixel electrode, wherein the slit mask has a plurality ofpatterns such that the slit mask has a blocking portion, a diffractionexposure portion and a complete exposure portion, and wherein theplurality of patterns are positioned in the slit mask such that anamount of diffraction is substantially uniform in the diffractionexposure portion.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 is a plane view showing a unit pixel of a related art LCD;

FIG. 2 is a cross-sectional view showing a liquid crystal panel in therelated art LCD;

FIGS. 3A˜3E are processing views illustrating the fabrication processesof the related art LCD;

FIG. 4A is a view showing a pattern of a slit mask for forming a thinfilm transistor channel in the related art LCD;

FIG. 4B is a photograph showing a channel inferiority of a thin filmtransistor fabricated by the slit mask of the related art LCD;

FIG. 5A is a view showing a pattern of a slit mask for forming a thinfilm transistor pattern in an LCD according to the present invention;

FIG. 5B is a rough view showing a channel formation of a switchingdevice in the LCD according to the present invention;

FIGS. 6A–6G are processing views illustrating fabrication processes ofthe thin film transistor using a slit mask according to the presentinvention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings.

Light exposed through a slit mask has a very short wavelength, andtherefore, accuracy of a slit width is very important to generate adiffraction. According to the present invention, a switching device ofan LCD has a channel layer formed in a single direction by using a slitmask. The slit mask is arranged in a single direction in order to solvea problem that uneven exposure creates on a photoresist due to aninferiority of the slit mask, such as uneven etching or a channel layercut.

A structure of the slit mask according to the present invention will bedescribed with reference to FIG. 5. A slit mask according to the presentinvention includes: a source electrode pattern unit 501 having a ‘U’shape for forming a source electrode having a ‘U’ shape; a drainelectrode pattern unit 502 having a straight shape entered into aconcave portion of the source electrode pattern unit and maintaining apredetermined distance from the source electrode pattern unit; and aslit pattern unit 503 parallel with the major axis of the drainelectrode pattern unit and disposed on both sides of the major axisbetween the source electrode pattern unit 501 and the drain electrodepattern unit 502. These slit patterns are parallel with each other andhave a same size. In the slit mask, a length of the ‘U’ shape channel isabout 30 μm, and a separation distance between the source electrodepattern unit 501 and the drain electrode pattern unit 502 is about 51μm. A width of the slit is about 1 μm.

As shown in FIG. 5A, the slit pattern unit of the present invention isformed on both sides of the drain electrode pattern unit without a bentportion. Therefore, the channel layer is only formed on the left andright sides of the major axis direction of the drain electrode. Althoughthe length of the channel is slightly shorter than that of the channelformed by the related art slit mask, this has little effect on thefunction of the thin film transistor.

FIG. 5B is a view showing a switching device form by using a slit maskaccording to the present invention. The switching device comprises asource electrode having a ‘U’ shape and a drain electrode having a ‘□’shape formed by the slit mask of the present invention, and the channelarea 510 is formed between a long side of the drain electrode and thesource electrode. The channel is not formed between the short side ofthe drain electrode and the source electrode, as illustrated in FIG. 5B.

A method for forming a switching device of an LCD according to thepresent invention by applying the above mask will be described asfollows. FIGS. 6A–6G are cross-sectional views taken along a line II–II′in FIG. 5A, illustrating fabricating processes for a switching device.First, as shown in FIG. 6A, an electrode material is formed on thesubstrate 601, and then, a photolithography process is performed using afirst mask (not shown) to pattern a gate line and a gate electrode 602on the substrate 601.

After forming the gate line, as shown in FIG. 6B, a gate insulatinglayer 603 of SiNx or SiO₂ material, an active layer and a conductivelayer 606 made of a conductive material are sequentially formed. At thattime, the active layer is an amorphous silicon (a-Si) layer 605 and ahigh doped n-type layer 605. Also, source and drain electrodes will beformed on the conductive layer 606.

Next, as shown in FIG. 6C, a photo resist 607 is formed on theconductive layer 606, and after that, a photolithography process isperformed using a second mask (M), which is the slit mask of the presentinvention, to form a pattern of the photoresist (PR) which isselectively remained on the channel area.

As a result of the above process, a diffraction exposure is made betweenthe source electrode unit and the long side of the drain electrode toform a photoresist pattern having a stepped portion, and completeexposure is made between the source electrode unit and the short side ofthe drain electrode unit. Then, part of the photoresist is removed inthe PR developing process to expose the conductive layer 606.

FIG. 6D is a view showing an etching process after completing the abovedevelopment process with the mask. As the PR is removed through thedevelopment process, the area except the exposed active area, theconductive layer 606 disposed on an exposed area between the short sideof the drain electrode unit and the source electrode, the high dopedn-type layer 605 and the a-si layer 604 are etched and removed.Therefore, the photoresist pattern having the stepped portion formed bythe diffraction exposure using the slit mask remains on the source/drainelectrodes, as shown in FIG. 6D.

Next, an ashing process is performed on the stepped PR pattern to removea part of the PR and to expose the high doped n-type layer between thesource and drain electrodes. FIG. 6E shows that the high doped n-typelayer between the source and drain electrodes is exposed.

Next, as shown in FIG. 6F, the conductive layer 606, which is exposedbecause the PR pattern is selectively removed, and the active layer 604and 605 are etched as much as a predetermined thickness to separatecompletely the source/drain areas, and then, the remaining PR 608 isremoved. At that time, the active layer between the source electrode andthe long side of the drain electrode remains, while the active layerbetween the source electrode and the short direction of the drainelectrode is removed so as not to form the channel.

Next, as shown in FIG. 6Q a passivation layer 609 of SiNx material isformed on the entire surface. Next, a contact hole is formed using aphotolithography process, and then, a pixel electrode is formed tocomplete the switching device.

According to the fabrication method of the LCD of the present invention,which fabricates thin film transistors using a slit mask, the structureof the slit mask is constructed to include a slit pattern unit of asingle direction. This is to fabricate thin film transistors having auniform channel, and therefore, the device property can be made even.Also, the short circuit problem on the channel due to the line widthinferiority can be solved, and yield can be improved.

It will be apparent to those skilled in the art that variousmodifications and variation can be made in the present invention withoutdeparting from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A fabrication method of a switching device for a liquid crystaldisplay (LCD) comprising: forming a gate electrode on a substrate;forming an insulating layer, an amorphous silicon layer, a high dopedn-type layer and a conductive layer; forming source/drain electrodes;forming a channel layer by applying a single direction slit mask,wherein the single direction slit mask includes a source electrodepattern unit having a ‘U’ shape, a drain electrode pattern unit formedon a concave portion of the source electrode pattern unit and a slitpattern unit being substantially free of a bent portion inside theconcave portion and aligned in a single direction between the sourceelectrode pattern unit and the drain electrode pattern unit; forming apassivation layer; and forming a pixel electrode.
 2. The method of claim1, wherein the slit pattern unit is formed to be parallel with a longdirection of the drain electrode pattern unit.
 3. The method of claim 1,wherein the slit pattern unit is formed on both sides of the drainelectrode pattern unit.
 4. A fabrication method of a switching device inan LCD comprising: forming a gate electrode on a substrate; forming agate insulating layer, an active layer and a conductive layersequentially on the substrate, the active layer having an amorphoussilicon layer and a doped layer; forming a source electrode of ‘U’ shapeand a drain electrode formed on a concave portion of the sourceelectrode; forming a channel area of the switching device only between along direction of the drain electrode and the source electrode byremoving the active layer between a short direction of the drainelectrode and the source electrode; and forming a passivation layer anda pixel electrode.
 5. The method of claim 4, wherein the channel area isformed by applying a slit mask which comprises a source electrodepattern unit having a ‘U’ shape and a drain electrode pattern unithaving a straight line shape formed on a concave portion of the sourceelectrode pattern unit of ‘U’ shape and a slit pattern unit aligned in asingle direction between the source electrode pattern unit and the drainelectrode pattern unit.
 6. The method of claim 4, wherein the channelarea is symmetric as centering around the drain electrode.
 7. Afabrication method of a thin film transistor for a liquid crystaldisplay comprising: forming a gate electrode on a substrate; forming agate insulating layer, an amorphous silicon layer, a high doped n typelayer and a conductive layer sequentially on the substrate; formingsource/drain electrodes and a channel area using a slit mask; andforming a pixel electrode, wherein the slit mask has a plurality ofpatterns such that the slit mask has a blocking portion, a diffractionexposure portion and a complete exposure portion, wherein the pluralityof patterns comprises a source electrode pattern having a ‘U’ shape, adrain electrode pattern formed on a concave portion of the sourceelectrode pattern and a slit pattern being substantially free of a bentportion inside the concave portion and aligned in a single directionbetween the source electrode pattern and the drain electrode pattern,and wherein the plurality of patterns are positioned in the slit masksuch that an amount of diffraction is substantially uniform in thediffraction exposure portion.
 8. The method of claim 7, wherein the slitpattern is formed to be parallel with a long direction of the drainelectrode pattern.
 9. The method of claim 7, wherein the slit pattern isformed on both sides of the drain electrode pattern.